Alamosaurus
sanjuanensis lived during the Late Cretaceous Period about 73-65 million
years ago in what is now North America. This large sauropod was perhaps
the last giant herbivore to live in the North American West. Giant sauropods
dominated the American West at the end of the Jurassic Period, 145 million
years ago. They seem to be absent from North America for at least 25 million
years ago.

Alamosaurus's body was as large as that of Apatosaurus, its tail, however,
seems to have been a few yards shorter than the famous Apatosaurus whip
like tail. It weighed about 30 tons, as much as 6 elephants. Alamosaurus
was descended from the South American dinosaurs that moved northward once
the landmasses of the Americas were reunited near the end of the dinosaurs
about 65 million years ago. Some relatives had armor on their bodies,
but it is not known if Alamosaurus shared that trait.